1. Field of the Invention
This invention relates to a non contacting measurement of the distance from an object to a reference point.
2. Prior Art
Many methods are known for determining the position or distance of an object from a reference point. Frequently, these same methods may be used to determine whether an object is present at a position (or a range of positions) or the temporal rate at which an object appears at a position (e.g., as for a rotating or oscillating object).
These methods may be divided into contacting and noncontacting types depending on whether the object, or an extension of the object, contacts the sensing element. A well known contacting method involves measuring a change in electrical resistance and is illustrated in FIG. 1. An extension 11 of an object 10 is an electrical conductor like a metal which slides on a second conductor 13 as the object moves. The change in contact position between the two conductors varies the length and, accordingly, the resistance, R.sub.v, of that portion of conductor 13 appearing in an external circuit 14. Element 11 is joined to external circuit 14 by a flexible wire. The variation of resistance with object motion affects the electrical characteristics of external circuit 14 in a manner convenient for measurement, thus providing a sensing of the position of object 10. Although useful in some applications, the contact between element 11 and conductor 13 can be subject to wear, vibration (leading to electrical jitter) or chemical contamination depending on ambient conditions.
Of the numerous types of noncontacting position and/or pick-up sensors, many involve electromagnetic energy in the form of capacitive, magnetic (e.g., linear variable differential transformer, Hall effect) or optical methods. Acoustical methods (e.g., sonar, ultrasonic) are also widely used. Each of these methods has its particular area of applicability arising from consideration of cost, durability, operating environment, etc.
Also, methods using a resonantly vibrating element in combination with electromagnetic techniques are especially advantageous for use in the automotive environment. U.S. Pat. No. 4,297,872 to Ikeda et al describes a vibration type transducer having a vibrator (e.g., a hollow metal cylinder) and vibration exciters (e.g., piezoelectric elements) which with suitable electrical activation cause the vibrator to vibrate in one or more of its resonant modes. Vibration detection means are located on or near the vibrator to sense the motion and provide an electrical output. In operation, a material or object whose property is to be sensed is suitably placed in proximity to the vibrator so that it modifies the resonant vibrational frequencies. For example, a fluid whose pressure is to be sensed is introduced into the cylinder. The vibrator may be so designed that the change in resonant frequency attending the introduction of this fluid is proportional to the fluid pressure. Alternately, the temperature of the fluid may alter the resonance in a characteristic way. Different methods may be used so that the approach of an object (e.g., its position) causes this resonance frequency to change in a particular way.
Vibration detection means can be used in two ways. First, they serve as an input to feedback electronic circuitry whose output is applied to the vibration exciters to keep the vibration excited at its resonant frequencies even though those frequencies may be changing. Secondly, they serve as input to additional circuitry for processing the frequency information so that an electrical output related to the quantity to be sensed is produced. Such a device typifies one method of operating vibrational sensors in which the quantity of interest modifies the vibrators resonant frequency. One of the disadvantages of this approach is that sensitivity can be low because the frequency does not go to zero, but rather returns to some fiducial value, as the perturbation or change which causes the frequency variation is reduced to zero.
This disadvantage does not occur in the present case, and an embodiment of this invention has high sensitivity and dynamic range. In addition, the device is appropriate for low cost manufacture as well as the ambient conditions peculiar to the automotive environment.